In a positron emission tomography (PET) imaging system, a radionuclide is injected into a subject of interest. As the radionuclide decays, positrons are emitted that collide with electrons, resulting in an annihilation event that emits pairs of gamma particles. The pairs of gamma particles impact a detector array, which allows localization of the origin of the annihilation event. After a series of events are detected, localized concentrations of the radionuclide can be ascertained, leading to a functional diagnostic image.
As PET data is acquired, attenuation of some of the emitted photons may occur. Attenuation, or the phenomenon of reduction in the number of photons detected as compared to the amount of photons emitted, can lead to degraded image quality and reduced quantitative accuracy. Accordingly, in certain situations, such as patient imaging, PET imaging is combined with X-ray computed tomography (CT) imaging to correct for such attenuation. Because CT imaging is based on the attenuation of X-rays by the imaged target, the CT image can provide information relating directly to the attenuation coefficients of the materials (e.g., tissues) being imaged. To perform such correction, attenuation values are mapped to the CT image, and the effective energy used to generate the CT image is translated to PET energies.